Local electronic density and vacancy properties for YBa2Cu3-xMxOy (M = Al, Sn) ceramic materials

In this paper, we report the positron lifetime data in high-T-c cuprate perovskite YBa2Cu3-xMxOy (M = Al, Sn) systems with different dopant concentration x. A significant substitution dependence of the positron lifetime parameters was observed, and the local electron density n(c) and defect concentration C-v were also evaluated as a function of substitution content x. The results show that in the region of known structural transition from orthorhombic to tetragonal phase, n(c) and C-v present an large anomalous change. Here, n(e), is decreased and C-v increases. These can be interpreted by electronic weak localization in Cu-O chains and would affect on the charge transfer in superconducting state. The mechanisms of positron annihilation were also discussed. (C) 1998 Elsevier Science B.V. All rights reserved.